MariposasCarb Loading Calculator
By Zachary, builder of Mariposas · Reviewed against published exercise-science sources (references)
· Last updated June 2026
Nutrition & Calories General endurance guidance (8-12 g/kg) for the 1-2 days before a long event.
Estimate how many grams of carbohydrate to aim for in the day or two before a long endurance event, based on your body weight and general sports-nutrition guidance.
⚕️ A general-information estimate from population-level formulas, a starting point, not a precise measurement and not medical advice.
How it works
Carb loading calculators draw on the body-weight-based targets established in sports nutrition research, typically expressed as grams of carbohydrate per kilogram of body weight per day. The most commonly cited loading range for endurance athletes runs from roughly 8 to 12 g per kg per day during the one to two days before a long event, with the higher end of that range associated with classic glycogen supercompensation protocols studied in the 1970s and refined since. The logic is straightforward: skeletal muscle can store only a limited amount of glycogen, and that ceiling is roughly proportional to muscle mass, which correlates with body weight. By flooding the system with carbohydrate while drastically cutting training volume, the muscles absorb more glycogen than they would under normal fueling, effectively extending the point at which you hit the wall. The calculator multiplies your body weight in kilograms by a target loading factor to give you a daily gram target, which you can then spread across meals and snacks.
When to use it
This tool is most relevant for athletes preparing for continuous endurance events lasting roughly 90 minutes or longer, such as marathons, long triathlons, century rides, or long-course open-water swims, where glycogen depletion is a realistic limiter on performance. It helps answer a practical planning question: how much food actually needs to go on the plate across the loading day, rather than just eating "a lot of carbs" without any reference point. Strength athletes, CrossFit competitors, or people doing shorter events generally do not need this kind of calculation, because glycogen stores are rarely the bottleneck in efforts lasting under an hour.
Worked example
Say someone weighs 75 kg and is running a marathon on Sunday. Using a moderate loading target of 10 g per kg, the calculator returns 750 g of carbohydrate for Saturday, the primary loading day. To put that in food terms, 750 g of carbohydrate works out to roughly 12 to 13 medium bagels, or a combination of rice, pasta, bread, bananas, and sports drinks spread across four to five meals. That number also clarifies the calorie math: 750 g of carbohydrate contributes about 3,000 calories from carbs alone, so total daily intake on a loading day is substantially higher than a normal training day, which surprises many athletes the first time they actually calculate it.
Tips for an accurate result
- Enter your weight in kilograms if the calculator offers both units, since all the underlying research targets are expressed per kg and unit-conversion rounding can add up over a large body weight.
- Focus the carbohydrate sources on low-fiber, easily digestible options like white rice, white bread, sports drinks, and ripe bananas. High-fiber whole grains technically count toward the gram total but raise GI distress risk the day before a race.
- Spread the carbohydrate across at least four meals rather than trying to hit the gram target in two or three sittings. Gastric discomfort and bloating are common when athletes try to front-load everything into a single enormous pasta dinner.
- Reduce fat and protein slightly on the loading day to make room for the high carbohydrate volume without forcing total calorie intake to an uncomfortable extreme. The goal is carbohydrate density, not maximum caloric excess.
- Use the output as a planning anchor the week before the event, not just the night before. Many athletes start modestly increasing carbohydrate intake two to three days out and ramp to the full target on the final full day before competition.
Formula & sources: methodology · references.
Now go hit the number Mariposas turns every workout, run and class into progress · collect a cute pet 🐾FAQ
- Does carb loading actually work, or is it overhyped?
- The evidence is reasonably solid for events lasting 90 minutes or more. Multiple controlled trials have shown measurable improvements in time-to-exhaustion and time-trial performance when athletes complete a proper loading protocol versus going in with normal glycogen levels. The effect is smaller or absent in shorter efforts, which is why the duration threshold matters.
- Will I gain weight from carb loading and does that hurt performance?
- Yes, body weight typically rises by 1 to 3 kg during a loading protocol. Each gram of glycogen is stored alongside roughly 3 to 4 grams of water, so a meaningful glycogen increase brings noticeable water retention. For most endurance events this is a worthwhile trade, since the extra glycogen availability outweighs the minor weight increase. The temporary water weight disappears over the days following the event.
- Can I carb load the morning of the event instead of the day before?
- A pre-race breakfast does contribute to liver glycogen and blood glucose, but a single morning meal cannot replicate what 24 hours of elevated carbohydrate intake achieves in muscle glycogen stores. The morning meal is a useful complement, not a substitute. Most sports dietitians describe the loading day as the critical window, with the pre-race meal serving as a top-off.
- What if the gram target feels impossibly high to eat?
- It often is, especially for smaller athletes or those not used to high-carbohydrate eating. Liquid carbohydrate sources like sports drinks, juice, and smoothies make it easier to reach the target without feeling stuffed. Starting the loading protocol a day earlier at a more moderate intake level is another practical workaround many athletes use successfully.
- Does the same logic apply to cycling, swimming, or rowing, or just running?
- The glycogen depletion mechanism is essentially the same across all aerobic modalities that sustain a moderate to high intensity for 90-plus minutes. The sport matters less than the duration and the metabolic demand. A 3-hour open-water swim or a 5-hour cycling sportive draws on muscle glycogen heavily enough that a loading protocol is well-supported by the same underlying physiology.